The present invention relates to the field of hydraulic pumps, and more particularly to axial piston pumps of the type having intersecting axes.
In general, axial piston pumps in common use comprise a body containing a rotor mounted to rotate about its central axis and presenting a series of axial bores each slidably receiving a piston having one end constrained to remain substantially in a fixed plane that is not perpendicular to the axis of the rotor, thereby causing the piston to perform reciprocating motion in the bores while said rotor is rotating.
The bores and their pistons thus define variable-volume cylinders in communication with suction and delivery ports of the pump via a distribution seat that is mounted stationary in the pump body and that co-operates with the axial bearing face of the rotor, through which the axial bores open out. This distribution seat presents two kidney-shaped openings that are diametrically opposite and connected respectively to the suction port and to the delivery port.
Document FR-A-1 261 358 thus describes a pump having a rotor but not of the intersecting axis type. The rotor of that pump comprises two coaxial bodies that are rigidly secured to each other by means of fixing bolts and centering pins. The spring causing the rotor to bear against the distribution seat is interposed between a shoulder of the bottom body and the central cap of a rotating and oscillating cover, itself interposed between a central ball and hemispherical bearing surfaces formed on the axial pistons. Such an arrangement gives rise to high levels of friction that prevent high speeds of rotation.
Document FR-A-1 456 563 describes a pump in which the rotor is urged against the distribution seat by a resilient washer having a conical bearing surface bearing against a corresponding shoulder provided on the support for the distribution seat, so friction is high. The rotor is additionally centered by means of a central tube secured to the support of the distribution seat, so the only function of the driver is to rotate the rotor without being involved in centering the rotor or pressing said rotor against the distribution seat. The structure of such a pump is extremely complex, and implies high levels of friction that put a limit on operating speeds.
Document NL-A-248 888 illustrates a pump having intersecting axes in which the rotor is rotated by a driver screwed to the pump, and centered on a fixed axis. Thrust against the distribution seat is provided by Belleville washers bearing against a ring mounted on a central shaft secured to the support for the distribution seat. In that case also, the driver serves only to rotate the rotor.
Although it is now possible to obtain bearing faces with a very high degree of planeness, machining tolerances, and in particular concerning the perpendicularity of the bearing surface to the axis of rotation of the rotor, mean that deformation in operation makes it impossible to guarantee that the rotor bears continuously over its entire bearing face against the distribution seat.
Furthermore, in order to prevent the end of a piston sliding against the swashplate defining the fixed plane, a conventional improvement for pumps of this type consists in causing the swashplate to rotate with the rotor about an axis normal to the fixed plane and intersecting the axis of the rotor, using a constant speed coupling, e.g. meshing bevel gears. Under such circumstances, a link is advantageously provided between each axial piston and the swashplate, with the link having a ball-and-socket joint at both ends. This eliminates all sliding friction between the moving parts, other than sliding on the piston in its bore.
Still in the context of this improvement, the rotor is urged axially against the distribution seat in conventional manner by means of a spring bearing against the central zone of the swashplate, e.g. via a central ball secured to the swashplate, and urging the rotor against the distribution seat. The spring is thus installed between two parts that move relative to each other, thereby giving rise to sliding friction between the spring and at least one of those parts, subjecting them to wear and causing particles to be given off that pollute the inside of the pump.
In addition, the force of the spring is additional to the force of the links forcing the pistons into the bores in the rotor, giving rise to a high level of force on the swashplate, and that can overload the bearings concerned.
An object of the invention is to mitigate the above drawbacks by proposing an axial piston hydraulic pump of the intersecting axis type which does not have the above-mentioned drawbacks and limitations.
This problem is solved by a hydraulic pump having axial pistons, the pump being of the type having intersecting axes, and comprising a rotary rotor with a central axis co-operating with an associated distribution seat via a bearing surface that is substantially perpendicular to said axis, said rotor presenting a plurality of axial bores slidably receiving corresponding pistons, the pump being remarkable in that a driver separate from the rotor is provided to rotate about an axis of rotation that essentially coincides with said central axis, said driver serving firstly to center and rotate the rotor about said axis of rotation, and secondly to apply resilient thrust to press the bearing surface of the rotor against the distribution seat.
By being disunited from the driver in this way, the rotor is free during rotation to move axially and angularly under the effect of the spring to compensate for alignment defects between its own bearing face and the distribution seat. Furthermore, the thrust force of the spring is taken up by the driver, thereby avoiding stressing any other part of the pump. In addition, the spring is installed between two parts that move very little relative to each other, thus avoiding any sliding friction between the spring and either of those parts.
According to an advantageous characteristic, the driver receives the rotor in a hollow portion, a bearing surface formed inside the driver centering the rotor on the axis of rotation, while leaving the rotor free to slide axially relative to the driver.
Advantageously, the bearing surface is situated axially substantially halfway along the rotor.
As a result, the parasitic radial forces due to the pistons are countered substantially where they are introduced.
Preferably, the centering leaves the rotor with a certain amount of freedom to move angularly relative to the driver.
This leaves the rotor free to move angularly so as to remain continuously well pressed against the distribution seat.
According to another advantageous characteristic, the rotor and the driver are linked in rotation by corresponding mechanical means of complementary shapes carried by said rotor and driver. This can be provided by co-operation between two symmetrical lugs formed on one of the elements and complementary cavities formed in the other element, or in a variant by co-operation between fluting formed on one of the elements at the bearing surface and complementary fluting formed on the other element.
The rotary drive imparted in this way puts pure torque on the rotor, thereby leaving the rotor free to move relative to the driver.
Also advantageously, the driver has a transverse wall substantially perpendicular to its axis of rotation, with resilient means compressed against an inside face thereof to thrust the rotor against the distribution seat.
This wall thus counters the force of the resilient means and transmits it to the pump body via the rotary connection between the driver to the body (which connection can be provided for example by means of roller bearings), without this force stressing other portions of the pump, or creating interfering friction.
In an important embodiment in which the axial pistons are hinged to a rotary swashplate whose axis of rotation intersects the axis of the driver, the swashplate is advantageously constrained to rotate with the driver, preferably by means of bevel gearing.
In which case, the parasitic force introduced by contact between the teeth of the bevel gearing in addition to the driving torque is filtered by the driver, thus sparing the rotor. Furthermore, the movements of the rotor do not disturb meshing conditions.
Advantageously, the swashplate is constrained to rotate with the drive shaft of the pump.
In a particular disposition, for a pump in which the axial pistons are hinged to the swashplate by means of links having spherical ends, the swashplate receives the ends of the links in associated shaped sockets which are held to the swashplate by means of a common circular coupling plate, itself connected to the swashplate by means of a central bolt.